1. Field of the Invention
The present invention relates to a polymer electrolyte containing inorganic conductive nano-particles and a fuel cell employing the polymer electrolyte, and more particularly, to a polymer electrolyte having excellent ionic conductivity and capable of effectively blocking flow of methanol by inorganic conductive nano-particles contained in the polymer electrolyte and a fuel cell employing the polymer electrolyte.
2. Description of the Related Art
Direct methanol fuel cells (DMFCs) directly supply a liquid fuel such as methanol to electrodes, so they need no additional equipment such as a fuel reformer or a hydrogen reservoir, which is required in polymer electrolyte fuel cells. DMFCs can be miniaturized to be readily applied under any urgent situation. Furthermore, DMFCs have a high energy density and is easily available as an environment-friendly, clean energy source. One of most important factors which affect the performance and cost of such DMFCs is a polymer electrolyte used therein.
In a fluorinated polymer membrane, which is widely used as a polymer membrane for a DMFC, methanol is liable to migrate while being hydrated, rather than be oxidized, so that cross-over of the methanol through the polymer membrane occurs. Greater cross-over of the methanol further reduces the potential of the fuel cell and hinders the reduction of oxygen and thus is considered to be the main cause of performance degradation of the fuel cell. Therefore, suppressing the cross-over of methanol is the first consideration for practical uses of the DMFC.
The DMFC needs a kind of barrier for blocking direct migration of the methanol between an anode and an cathode while allowing only protons to pass.
Even if a polymer membrane capable of acting as a barrier that passes only protons while suppressing the cross-over of the methanol is available, performance of the DMFC may degrade due to generation of over-voltage if proton conductivity of the polymer membrane is low.
U.S. Pat. No. 5,874,182 discloses a method of minimizing the effect of the cross-over of the methanol by using excess catalyst in the electrode. However, this method was unsatisfactory in suppressing the methanol's cross-over and is not considered to be a fundamental solution of the cross-over. As a result, many attempts have been made to improve the performance of the DMFC by improving the polymer electrolyte membrane itself so as to prevent the cross-over of methanol.
U.S. Pat. No. 5,795,668 discloses a method of reducing cross-over of methanol by supporting solid polymer electrolyte membranes against both sides of a porous support. U.S. Pat. No. 5,958,616 discloses a method of oxidizing methanol, with the addition of a catalyst, to an electrolyte membrane that is formed to incorporate channel or path for methanol therein. However, this method increased the cross-over of methanol through the channel or path of the electrolyte membrane. In addition, the use of the catalyst and the porous support further increases the cost of the solid electrolyte membrane, which is already expensive to produce, and thus the method is impractical.
U.S. Pat. No. 5,919,583 discloses a method of reducing cross-over in a DMFC by dispersing an inorganic material such as zeolite and zirconium in the polymer electrolyte. However, this method has the following problems. To incorporate such inorganic particles into the polymer electrolyte composition, there is a need to change the polymer electrolyte composition into an alkaline state, which is a complicated process. Furthermore, such simple dispersion of the inorganic particles in the polymer electrolyte membrane is effective in preventing the methanol crossover but the proton conductivity is decrease. As described above, although approaches have been made in a variety of aspects to reduce the cross-over of methanol in the DMFC, those suggestions still have considerable technical limitations in the decrease proton conductivity.